Origin of a shallow electron pocket: $β$-band in Co$_{1/3}$TaS$_2$ studied by angle-resolved photoemission spectroscopy

Abstract

We investigated the electronic structure of Co-intercalated 2H-TaS$_2$ using angle-resolved photoemission spectroscopy (ARPES). In the compound Co$_{1/3}$TaS$_2$, the main electronic bands closely resemble those of pristine 2H-TaS$_2$, with no clear signs of band folding. However, a shallow electron pocket, referred to as the $β$-feature, was detected at the Fermi level near the corner of the superlattice Brillouin zone. The surface vs bulk origin of this feature is debated, as it cannot be reproduced using standard DFT calculations. To resolve this, we employed cluster perturbation theory (CPT) to incorporating an exact treatment of strong electron correlations (U) on the cobalt sites, going beyond DFT+U approximation. To further substantiate this, we studied an underdoped sample, Co$_{0.22}$TaS$_2$, where a reduced charge transfer leads to different Co orbital character near the Fermi level. We find that its electronic structure closely resembles that of undoped 2H-TaS$_2$, and crucially, lacks the $β$-feature. Our results demonstrate that the $β$-feature is of the bulk origin emerging from the strong electronic correlations where both the Co charge state and long-range crystallographic order play an important role. This work highlights the need for accurate treatment of electron correlations when studying intercalated transition metal dichalcogenides.

Figures (6)

• Figure 1: (Color online) (a) ARPES intensity plot of Co$_{1/3}$TaS$_2$ along $\overline{\mathrm{\Gamma}}{\overline{\mathrm{M}}}_0$ direction at 20 K measured with h$\nu$ = 72 eV. The vertical-dashed lines represent the boundaries of Co$_{1/3}$TaS$_2$ surface Brillouin zone. (b) The Fermi surface map at 20 K with h$\nu$ =72 eV. The black hexagon and the smaller blue hexagon shown in the dashed line represent the first Brillouin zone of 2H-TaS$_2$ and Co$_{1/3}$TaS$_2$, respectively.
• Figure 2: (Color online) (a) Graphic representation of the crystal structure. Co atoms are shown as green spheres, overlaid with the corresponding Co Wannier orbitals. Red isosurfaces represent the interstitial Ta Wannier orbital lobes located between neighboring Ta atoms rather than at the atomic positions themselves (shown as blue spheres). Intra-cluster connections are indicated by dashed green lines and the clusters are shaded in green. The unit cell is indicated by a thin black line and shaded in gray. Sulfur atoms coordinating Ta in trigonal prismatic arrangement are omitted for clarity. These sulfur atoms occupy positions such that the edges of the trigonal prisms, connecting sulfur atoms in the layers above and below, do not intersect the Ta Wannier orbitals; that is, the sulfur triangles are rotated by $30^\circ$ with respect to the triangles defined by the Wannier orbital positions. Comparisons of the spectra presented in Fig. \ref{['fig1']} with the results of the CPT calculation: (b) the band structure along $\overline{\mathrm{\Gamma}}{\overline{\mathrm{M}}}_0$ direction and (c) the Fermi surface map of Co$_{1/3}$TaS$_2$.
• Figure 3: (Color online) (a) ARPES intensity plot along the $k_z$ direction at the Fermi level. The vertical dashed lines correspond to the boundaries of the Co$_{1/3}$TaS$_2$ Brillouin zone. The horizontal green lines and labels ($\mathrm{\Gamma} _{n+1}$) mark the $k_z$ levels corresponding to $\mathrm{\Gamma}_{n+1}$$\equiv$ (0, 0, 2$\pi/c \times n$) . (b) The CPT calculated Fermi surface along $k_z$-direction.
• Figure 4: (Color online) (a) ARPES intensity plot of Co$_{0.22}$TaS$_2$ measured with h$\nu$=72 eV at 20 K. (b) The Fermi surface along $k_z$ direction obtained by changing photon energy from 57 to 90 eV with a step of 3 eV at 20 K. (c) The Fermi surface measured with h$\nu$=63 eV at 20 K.
• Figure 5: Comparison of the DFT band structure (dotted lines) with the band structures obtained from the Wannier Hamiltonians, (solid lines). The upper panel shows the band tructure constructed using 52 Wannier functions per unit cell, while the lower panel displays the result obtained with a reduced basis of 7 Wannier functions per unit cell.